1. Field of the Invention
The present invention relates to an apparatus and method for reducing the size of coal, minerals (including ores, compounds, and elements), biomass, waste, and other material. More specifically, the invention relates to a two-stage micronizing mill for reducing the size of such materials.
2. Related Art
Efficient size reduction technology through application of dual counter-rotating rotary mill design is undertaken in numerous patents. U.S. Pat. Nos. 5,275,631 and 5,575,824 to Brown et al. disclose combining such meanswith refuse separation means. U.S. Pat. No. 5,597,127 to Brown provides for finer milling. The present invention provides an improved method and apparatus for finer milling, and with particular regard to the problem of efficient reduction of oversize particles, provides for second stage selective milling integral to the mill itself, without classification and recirculation.
Classification and recirculation requires additional mechanical means which adds to capital costs. Operating costs increase also since typically discrimination is not precise and more material than necessary is returned for re-milling, including particles milled to within size specification as well as oversize particles.
The term "second-stage milling" refers to size-reduction by means of a separate type than that employed in the first stage. Here, primary milling is accomplished by attrition and impacting, while second stage reduction is accomplished by crushing, or--in the case of unfriable, fibrous materials--the crushing action results in pinching and rolling which separates fibers.